AQA Science © Nelson Thornes Ltd 2006  1

P3 1.2

What are we studying today?

AND

+ not mine but…

press yourPRESSURE

highdrawer licks HYDRAULICS

AQA Science © Nelson Thornes Ltd 2006  2

P3 1.2

http://www.youtube.com/watch?

feature=endscreen&v=qiFL_yOvV8Y&NR=1

Siri’s feeling the pressure

AQA Science © Nelson Thornes Ltd 2006  3

P3 1.2

Pressure and Hydraulics

Objectives

Be able to describe what pressure is and how it applied to hydraulic systems.

19 April 2023

P3 1.2

AQA Science © Nelson Thornes Ltd 2006  4

Outcomes• All MUST be able to describe

pressure is.• MOST Should be able to use the

equation for pressure. • SOME could be able to

rearrange the equation for pressure and use it in different situations.

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Definition:

• Hydraulics is the use of pressure in liquids. To understand hydraulics, first we must understand pressure.

• Pressure is the Force per Unit Area.

• Pressure (Pascals) = Force (N) / Area (m2)

Crushing Can demo: http://www.youtube.com/watch?v=0NZmo-qQxIk

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How a gas exerts pressure• A gas consists of molecules in

constant random motion.• When a molecule collides with a

surface it reverses direction due to the force exerted on it by the surface.

• The molecule in turn exerts a force back on the surface.

• The pressure exerted by the gas is equal to the total force exerted by the molecules on a particular area of the surface divided by the area.

• pressure = force / area

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Pressure examplespressure in Pa

or N/m2

Space (vacuum) 0

Air pressure at the top of Mount Everest

30 000

Average pressure of the Earth’s atmosphere at sea level at 0°C

101 325

Typical tyre pressure 180 000

Pressure 10m below the surface of the sea

200 000

Estimated pressure at the depth (3.8km) of the wreck of the Titanic

41 000 000

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Other pressure unitsNote: You do not need to learn any of these for the GCSE exam

Atmospheres (atm)Often used to measure the pressure of a gas. An atmosphere is the average pressure of the Earth’s atmosphere at sea-level at a temperature of 0°C.Standard atmospheric pressure = 101 325 Pa (about 101 kPa)

Bars and millibars (bar; mbar)Also used to measure gas pressure. One bar is about the same as one

atmosphere.

Millibars are often found on weather charts.

1000 millibars = 1 bar = 100 kPa

P3 1.2Pounds per square inch (psi)Often used to measure car tyre

pressures.1 psi = 6895 Pa

1 atm = 101 kPa = 14.7 psi

Inches of mercury (inHg)Often found on domestic barometers.

1 inHg = 3386 Pa1 atm = 101 kPa = 29.9 inHg

Examples:Fair weather – high pressure: 30.5 inHg

Rain – low pressure: 29.0 inHg

tyre pressure gauge

P3 1.2

Pressure and Hydraulics

AQA Science © Nelson Thornes Ltd 2006  10

• Watch:• http://www.youtube.com/watch?

v=6b3xBUxv0qE&playnext=1&list=PL1696A5429EF5BC33&feature=results_video

• Prepare to describe the 3 most important things you learn from this – you may want to jot points down in the back of your book

P3 1.2

Pressure, ppressure = force

area

p = F

A

units:

force, F – newtons (N)

area, A – metres squared (m2)

pressure, p – pascals (Pa)

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also:

force = pressure x area

and:

area = force

pressurep A

F

Note:1 Pa is the same as 1 newton per square metre (N/m2)

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Pressure exerted by a person on a floor

1. Weigh the person in newtons. This gives the downward force, F exerted on the floor.

2. Draw, on graph paper, the outline of the person’s feet or shoes.

3. Use the graph paper outlines to calculate the area of contact, A with the floor in metres squared.

(Note: 1m2 = 10 000 cm2)

4. Calculate the pressure in pascals using: p = F / A

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Typical results1. Weight of person: _____ N

2. Outline area of both feet in cm2 ____

3. Outline area of both feet in m2 _____

4. Pressure = ________

= _______ Pa

500

60

0.006

500 N0.006 m2

83 000

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Why off-road vehicles have large tyres or tracks

In both cases the area of contact with the ground is maximised.

This causes the pressure to be minimised as:

pressure = vehicle weight ÷ area

Lower pressure means that the vehicle does not sink into the ground.

P3 1.2

Question sheet – 15 minutes

• Work with your partner and complete as many questions as you can

Extension on next page

P3 1.2Pressure exerted by a block question - extension

The metal block, shown opposite, has a weight of 900 000N. Calculate the maximum and minimum pressures it can exert when placed on one of its surfaces.

Maximum pressure occurs when the block is placed on its smallest area surface (2m x 3m)p = F / A= 900 000N / 6m2

Maximum pressure = 150 000 Pa

Minimum pressure occurs when the block is placed on its largest area surface (3m x 5m)p = F / A= 900 000N / 15m2

Minimum pressure = 60 000 Pa

2m

5m3

m

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Question 1Calculate the pressure exerted by a force of 200N when applied over an area of 4m2.

p = F / A= 200N / 4m2 pressure = 50 Pa

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Question 2Calculate the force exerted by a gas of pressure 150 000 Pa on an object of surface area 3m2.

p = F / A

becomes:

F = p x A

= 150 000 Pa x 3 m2

force = 450 000 N

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Question 3Calculate the area that will experience a force of 6000N from a liquid exerting a pressure of 300kPa.

p = F / A becomes:A = F / p= 6000 N ÷ 300 kPa = 6000 N ÷ 300 000 Pa area = 0.02 m2

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Complete:

force area pressure

40 N 8 m2 Pa

500 N 20 m2 25 Pa

400 N 5 m2 80 Pa

20 N 2 cm2 100 kPa

6 N 2 mm2 3 MPa

5

20

400

100

2

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What can you do?

• I most definitely can describe what pressure is.• I should be able to apply my understanding and use

the equation for pressure.• I may be able to rearrange the equation for

pressure and use it in different situations.

• Tell me what you thought was easy about this lesson and what was difficult. Anything that would help you in the future?

• Eg: I thought that …. was easy/difficult because …• It would help if …